(1) Field of the Invention
The present invention relates to a solid-state imaging device having on-chip color filter layers and a manufacturing method of the solid-state imaging device.
(2) Description of the Related Art
With the advancement of a color image forming technique, recent years have seen a remarkable growth in the use of a single-board color type solid-state imaging device, the use being for a digital still camera mainly including CCD (Charge Coupled Device) type, and a mobile phone with a camera mainly including CMOS type. This leads to the increase in the demands for downsizing such a solid-state imaging device having on-chip color filters and the day-by-day increase in the number of pixels. However, in order to meet such demands for the solid-state imaging device like this, the light-receiving area of a photoelectric conversion element 13, which is a light-receiving sensor, must be downsized. This is becoming a cause for deteriorating photoelectric conversion characteristics (light sensitivity) which are a primal characteristic of a solid-state imaging device.
For example, main optical sizes of solid-state imaging devices to be mounted on digital still camera ranges from a third inch to a fourth inch, and further downsizing to a sixth inch or to below a sixth inch is being considered. Also, the number of pixels is becoming greater up to the range from 2 M pixels to 5 M pixels, and it is considered to increase the number exceeding 5 M pixels. Therefore, there emerges a need to establish a technique for maintaining the primal characteristics of a solid-state imaging device such as light sensitivity, color mixture between adjacent pixels and nonuniform tone of lines even in the case of downsizing a light-receiving area and increasing the number of pixels.
Here will be provided detailed description of this. Increasing the number of pixels without downsizing the pixel size causes the increase in the chip size, resulting in making the size of a solid-state imaging device larger. This means that the downsizing of the pixel size must be performed in parallel. In general, downsizing the pixel size leads to downsizing a photoelectric conversion element 13 represented by a photodiode, resulting in the deterioration in light sensitivity. In order to improve light sensitivity, a number of countermeasures have been taken. Especially, there have been proposed a number of structures, manufacturing methods and the like concerning a microlens which is formed on such a pixel.
Also, downsizing the pixel size becomes a cause for deteriorating not only light sensitivity but also various color characteristics stemming from a color filter layer. In general, making the pixel size finer causes deterioration in the dimensional accuracy of a color filter, resulting in the deterioration in characteristics such as color mixture between pixel filters which are adjacent to each other, nonuniform tone of lines, and sensitivity variations between pixel filters.
Therefore, the importance of on-chip color filter layers in a solid-state imaging device is increasingly becoming greater, and thus there is a demand for establishing a technique with which the deterioration of the characteristics such as color mixture, nonuniform tone of lines and sensitivity variations can be prevented.
FIG. 1 to FIG. 3 each depicts a sectional view of a pixel of a conventional solid-state imaging device.
These solid-state imaging devices 30 and 40 are each formed in the following way: forming a P-type semiconductor well region 12, which becomes second electric conductive type, on a semiconductor substrate 11 which is made of a first electric conductive type (for example, N type) silicon semiconductor; and then forming an N-type semiconductor region on the P-type semiconductor well region 12, a N-type semiconductor region and a P-type semiconductor well region 12 constitute a photoelectric conversion element 13. The respective photoelectric conversion elements 13 are formed in array shapes and arranged in a matrix form.
Further, on the boundaries of photoelectric conversion elements 13, for example, a conversion electrode 15 which is made of polycrystalline silicon is formed through the gate insulation film 14. On the conversion electrode 15 an inter-layer insulation film 16 which covers this conversion electrode 15 is formed. Also, on the rest of the whole surface, in other words, on the inter-layer insulation film 16 excluding the apertures of the photoelectric conversion elements 13, for example, a light-shielding film 17 which is made of AL, W and the like is formed. After that, the light-shielding film 17 and the gate insulation film 14 are covered with a surface protection film 18.
Further, a first transparent planarization film 19 is filled with each concave part above a photoelectric conversion element 13. After that, in the case of a solid-state imaging device 30, color filter layers 31G, 31B and 31R are respectively formed on each photoelectric conversion element 13. In contrast, in the case of a solid-state imaging device 40, color filter layers 41G, 41B and 41R are respectively formed on each photoelectric conversion element 13. In both the cases, a second transparent planarization film 42 is formed on the color filter layers 31G, 31B and 31R, and also the color filter layers 41G, 41B and 41R, and on-chip microlenses 43 are formed on the second transparent planarization film 42, the on-chip microlenses 43 collecting incident light to the respectively corresponding photoelectric conversion elements 13.
The first transparent planarization film 19 is for forming stable color filter layers 31 and 41 and for making the ground flat. The second transparent planarization film 32 is for planarizing the color filter layers 31 and 41 as the ground layer so as to form on-chip microlenses 43 accurately.
Such color filter layers 31 and 41 are made of one of the following two types of color filter layers: primary color filter layers which are made of red, green and blue filters; and complementary color filter layers which are made of yellow, cyan and magenta filters.
Also, the material used for the color filter layers 31 is pigment dispersion type and has excellent light-resistance and heat resistance. A representative of such a material is a mixture of: pigments; a dispersion agent; a photosensitive material; a resin; and the like. Color filter layers 31 are formed, according to a color resist method for obtaining a desired type of color filters, by performing a selective exposure process and a development process of a photo-resist film containing such a material. In this way, the solid-state imaging device 30 can provide the following two countermeasures for realizing high-definition against downsizing of pixels: slimming down a pigment filter which realizes improvement in the dimensional accuracy of color filter layers, and preventing nonuniform tone of lines, sensitivity variations and color mixture between color filter layers which are adjacent to each other; and making pigment particles finer. By means of the above-listed countermeasures, a color S/N ratio is being improved.
Also, the material which is used for the color filter layers 41 is made of a pigmented dye instead of a pigment and the like. The color filter layers 41 are formed, according to a color resist method for forming desired color filters, by performing a selective exposure process and a development process of a photo-resist film containing a mixture of such materials. Recent years have seen an accelerated research and development on this material, as a dye-containing color-resist which does not contain a fine particle, which can replace a pigment-dispersion color-resist. Some of such color-resists become commercially practical, in other words, some of them have been used for color filter layers of solid-state imaging devices (Refer to Reference Document 1: Japanese Laid-Open Patent Application No. 11-337715.).
The improvement example disclosed in the Patent Document 1 is for concurrently simplifying a manufacturing process and improving light-resistance and heat-resistance by means of making dyes into pigmented dyes in order to obtain desired spectral characteristics.
However, slimming down the pigment-dispersion filter used for a conventional solid-state imaging device 30 requires that a certain degree of film thickness be secured in order to obtain desired spectral characteristics, resulting in placing a restriction in slimming down. Further, making pigment particles finer involves a great difficulty, and it is impossible to prevent the diameter of secondary particles from increasing through re-aggregation even if those particles are once made finer. Further, since pigment particles are present as long as the pigment dispersion color filter layers 31 are employed, the problems which are caused depending on the dimensional accuracy of the color filter layers 31 have not yet been fundamentally solved, the problems being related to the color characteristics such as nonuniform tone of lines, color mixture, sensitivity variations and color S/N ratios.
This will be described below more specifically. The dimensional accuracy is improved because the following countermeasures concerning the pigment dispersion color filter layers 31 are taken: slimming down of a material which is used for a color filter (an increase in the pigment content); an improvement in resolution by means of such a material; and a countermeasure in the manufacturing process. However, there is a need to consider the influence of the particle size of a pigment which is used as a primal material of the color filter layers 31 in order to decrease the pixel size. Especially, since a pigment itself is considered to be a particle in dimensional accuracy and the secondary particle diameter is approximately 100 nm in general, great technical advancement is required to decrease the sizes of such particles down to 50 nm by taking a countermeasure of making such particles much finer. Also, whether or not desired spectrum characteristics can be obtained in the case of making particles finer has not yet been sufficiently confirmed. Further, since pigments are particles, it is inevitable that the taken image looks uneven and the color S/N ratios (signal to noise ratio) deteriorate even in the case where such a countermeasure of making pigment particles finer is taken. Therefore, assumingly, an existing technique places a limit to the use of such a pigment dispersion color resist which has a finer particle for color filter layers 31 in a solid-state imaging device.
In other words, it is difficult for us to cause a conventional solid-state imaging device 30 to prevent the deterioration in light sensitivity or color mixture between adjacent pixels only by slimming down such color filter layers accompanied by the reduction in the pixel size. The cause of color mixture will be described below more specifically. Color mixture occurs depending on the sectional edge shapes of color filter layers. The edges of the color filters cannot be cut vertically when they are formed. For this reason, a firstly-formed color filter layer has a trapezoid shape (the top surface dimension is smaller than the bottom surface dimension). Since a secondly-formed and thirdly-formed color filter layers are inserted into the gaps of the firstly-formed color filter pattern, they also have a trapezoid shape, in other words, the top surface dimensions are smaller than the bottom surface dimensions). Consequently, an oblique incident light passes through the edge parts of the color filter layers of adjacent pixels as shown in FIG. 1. This makes it impossible to obtain desired spectral characteristics, resulting in causing color mixture.
Also, there is a problem which makes pigment particles finer decreases the alignment margin because of a conventional sectional shape. This is because, in the case where a gap in arrangement is generated, oblique incident light which passes through the edges of the color filter layers of adjacent pixels as shown in FIG. 1 causes color mixture, which makes it impossible to obtain desired spectral characteristics. Further, color mixture degree varies depending on the angle of oblique incident light, and there are other problems such as nonuniform tone of lines, flicker, color shading and sensitivity variations.
In the case of using the pigment type color filter layers 31, exposure light randomly reflects on the surface of pigment particles and thus the light reaches comparatively deeper part of the filter. In contrast, in the case of the solid-state imaging device 40 for which the dye-containing color filter layers 41 are employed in order to improve the color S/N ratios, light polymerization reaction advances only in the proximity of the surfaces, in other words, the non-reaction parts are dominant inside the filters because there is no random reflection on the surfaces of the dye-containing type color filter layers 41.
A thermal process which is generally referred to as Post Exposure Bake (PEB) is performed after exposure in order to accelerate such reaction inside the filters. At this time, performing this process under an appropriate temperature is very important. This is because it is impossible to complete development in the case where the process is performed under a too high temperature, and because cavities are generated on the sectional surfaces after development in the case where the process is performed under a too low temperature (Refer to FIG. 2). Therefore it is general that PEB conditions (temperature and time) are determined considering these problems. However, conventional technique is not sufficient to solve these problems even though it can slightly improve the problem of cavities which are generated on the sectional surfaces.
An example of such cavities is shown as cavity β in FIG. 3. The cavity β is generated at the boundary of the first layer and the second layer when the material to become the second layer is coated. Consequently, light is diffused making the problems worse, these problems being color mixture, nonuniform tone of lines and sensitivity variations. Further, the consequent thermal process causes the gas in the cavity β to expand, which causes the deformation of the color filter layer 41, the transparent planalization film 42, and the microlens 43. This results in affecting the reliability of the device.